LCO-Based Proton Conducting Electrolyte And Related Materials Of Solid Oxide Fuel Cells

Solid oxide fuel cells?SOFC?are energy conversion devices that convert the chemical energy of fuel directly into electrical and thermal energy.It has the advantages of high energy conversion efficiency,high fuel versatility,no noise and low pollutant emission.High operating temperature is the biggest obstacle to the commercialization of SOFC.The application of proton conductor electrolyte as an effective means to reduce the operating temperature of SOFC has attracted extensive attention.In this paper,the La2Ce2O7-??LCO?materials with proton conductivity are applied to SOFC electrolytes.The research focuses on the development of SOFC key materials,the design of structures and the improvement of electrochemical properties.In Chapter 1,the research background,development and working principle of SOFC are briefly introduced,and the characteristics and preparation of current common SOFC electrolytes and electrode materials are emphatically described.In Chapter 2,the differences in conductivity of LCO doped with different alkaline earth metal elements?Mg,Ca,Sr?were first explored,and the relative advantages of Mg doped LCO were confirmed.Then,a series of studies were conducted on the optimal doping content of Mg elements in LCO.Experimental results show that all La2-xMgxCe2O7-??LMCO?ceramic powder is pure of fluorite structure.X=0.15 samples showed the highest conductivity,at 700 0? in the dry air and the wet hydrogen respectively reached 7.41×10-3 S cm-1 and 1.55×10-2 S cm-1.LMCO in the atmosphere of CO2 and H2O showed a good chemical stability.At the same time also found LMCO sintering activity increased significantly,at 1300 °C sintering 5 h got a LMCO completely dense electrolyte film.Corresponding single cell also shows a very encouraging electrochemical properties,at 700 ? peak power density of up to 897 mW cm-2.Through the study of Mg doped LCO,we developed a proton conductor material that is easy to sintering and has both chemical stability.The results show that LMCO as a proton conductor is a promising choice to achieve high performance output of SOFC under the condition of reducing the sintering temperature of electrolyte.In Chapter 3,a new structure of LCO based SOFC was designed for the short circuit phenomenon inside the cell caused by the reduction of Ce4+ to Ce3+ in the reducing atmosphere of LCO electrolyte.NiO-BZCY was used as the anode functional layer,and the cell structure was NiO-LMCO|NiO-BZCY|LMCO|SSC-SDC.NiO-LMCO and NiO-BZCY were used as LMCO-based SOFC anodes respectively.The influence of the thickness of the anode functional layer in the cell structure on the open circuit voltage and output power of the cell was studied.In addition to the catalytic function,the NiO-BZCY anode functional layer also acts as a Ba source,which is used to generate an electronic barrier layer in situ at the interface between the anode functional layer and the electrolyte in the sintering process.Electronic barrier layer thickness and NiO-BZCY anode layer thickness related function,with functions of 40?m thickness of NiO-BZCY anode layer of single cell open circuit voltage to 0.995 V?700 ??,at the same time the high power density of 830 mW cm-2,after 100 h of stability test,the cell power density and open circuit voltage is not obvious attenuation was observed.These results indicate that the new structure of LCO-based SOFC has a good application prospect.In Chapter 4,in the previous chapter,we found that the introduction of an electronic barrier layer in LCO-based SOFC can effectively increase the open circuit voltage of a single cell,but the ohmic resistance of the cell has significantly increased,indicating that the development of LCO series electrolytes material with high conductivity is urgently needed.In this chapter,we used alkali metal elements?Li,Na,K?to doping LCO system,and studied the influence of doping on the conductivity,sintering activity and cell power density of LCO series materials.In dry air,Lai.85Na0.15Ce2O7-??LNCO?shows the highest conductivity,and the LNCO electrolyte single cell also shows the highest power output in the subsequent electrochemical performance test,with the highest power density of 793 mW cm-2.It is interesting to note that Lai.85K0.15Ce2O7-?,in H2 atmosphere below 750 °C and the conductivity is higher than LNCO,at the same time in the H2 and dry wet H2 atmosphere showed significantly decreased the apparent activation energy of 0.609 and 0.667 eV,suggests Lai 85Ko.15Ce2O7-? under the reducing atmosphere with a larger number of proton transfer.The results show that the electrochemical properties of LCO can be significantly improved by doping alkali metal elements,and it is expected to be used in SOFC electrolyte materials.In Chapter 5,cathode material is an important factor limiting the development of proton-based SOFC.In this chapter,non-cobalt cathode materials are studied with iron and bismuth co-doping BaCeO₃-?.A series of single-phase cathode BaCeo.5Feo.5-xBixO₃-??x=0,0.1,0.2 and 0.3?were prepared.The effects of iron and bismuth doping ratio on phase structure and cathode properties were also discussed.Influence the cathode microstructure and single cell and exploration on the electrochemical performance of the sintering temperature,the optimum sintering temperature is 900 ?.With BaCe0.5Fe0.5-xBixO₃-? single cell cathode at 700 ?showed the peak power density is 943 mW cm-2 and the polarization resistance of 0.061 ? cm2.Good electrochemical properties and excellent long-term stability indicate that BaCe0.5Fe0.3Bi0.2O₃-? is an ideal proton conductor based SOFC cathode material.In Chapter 6,the improved phase-inversion tape casting method was applied to the same type of cell structure as in chapter 3,and the influence of two asymmetric NiO-BZCY anode support structures on the electrochemical properties of SOFC was studied.The results show that the microstructure of the anode support has a significant effect on the electrochemical properties of the cell.Composed of sponge-like layer and finger-like holes on cell anode support has better electrochemical performance,at 650 ? maximum power density is 823 mW cm-2.Compared with the batteries prepared by the traditional dry pressing method,the performance of the batteries is greatly improved.It is proved that the improved phase-inversion tape casting method is a better method for preparing SOFC anode supports.In chapter 7,the work of this paper is sorted out and summarized,and the future research direction of LCO-based SOFC is prospected.